The present invention relates to an image reading device for a facsimile apparatus or the like.
In an image reading device, or image reader, installed in a facsimile apparatus and others to serve as image inputting means, images in a document which are alinged with a predetermined readout line are focused to a line image sensor which implements main scanning, while the document is fed in a subscan direction by a document feed mechanism. Generally, the level of light incident to the line image sensor is not even along the length of the sensor and is lowered at opposite end portions of the sensor due to the characteristic intensity distribution of a light source, focusing characteristics of optics, and other factors. In light of this, a wave shaping circuit is used to shade outputs of the line image sensor for compensation. Specifically, outputs of the line image sensor generated by reading a reference or white image is stored in a line memory pixel by pixel as reference signals and, during readout of an actual document, outputs of the sensor are shaded referencing the stored pixel-by-pixel reference signals.
The problem with the above-described prior art wave shaping circuit, or wave shaper, is that contamination such as dust deposited on the reference image is reflected by a decrease in the level of an analog video signal representative of a particular pixel which is aligned with the contamination, i.e. output level of the line image sensor. Should signals effected by such contamination be falsely stored in the line memory as reference signals, images read out by referencing the reference signals would involve white lines and black lines.